This invention relates to a speed ratio control system for a continuously variable transmission for a vehicle, and more particularly to improvements in a speed ratio control system. An oil flowrate to a hydraulic servo of either an input or an output V-shaped pulley device is varied by a flow control value, so that a speed ratio between the input and the output sides can be changed.
In the past, a belt-driven continuously variable transmission have been used as automatic transmissions for vehicles. In general, these continuously variable transmission mechanisms have V-shaped pulley devices, each including a stationary pulley and a movable pulley which cooperate to create an effective diameter which is variable. Hydraulic servo devices are used to move the movable pulley. The V-shaped pulley devices are provided on input and output shafts with a driving belt extending therebetween so that rotation of the input shaft can be transferred to the output shaft. Normally, an oil flowrate to the hydraulic servo device on the input side is adjusted by a flow control valve, whereby the effective diameter of the V-shaped pulley device on the input side is changed. The hydraulic pressure of the hydraulic servo device on the output side is varied by a pressure control valve to thereby follow the change of the effective diameter of the V-shaped pulley device on the input side, so that the driving belt does not slip in transmitting the torque.
As compared with an automatic transmission mechanism consisting of a torque converter with groups of planetary gear units, the above-described continuously variable transmission mechanism is advantageous in that these are for abrupt changes in driving force during running of the vehicle, shift shocks are low, and the fuel consumption rate is good. In recent years, demand has increased for further improvements in the continuously variable transmission mechanism.
The speed ratio control of the continuously variable transmission has heretofore been performed as described below. First, a target value (normally, a target rotational speed Nin.degree. or a target speed ratio e.degree.) is calculated, a deviation D=Nin.degree.-Nin (or D=e.degree.-e) is calculated, and, in accordance with this deviation D, an oil flowrate Q (=a control voltage Vin of the flow control value, being commensurate to the oil flowrate Q) to the hydraulic servo device on the input side is calculated through an equation Vin=K.multidot.D. This K is a feedback gain and has heretofore been set at a constant value.
However, because of non-linearity of the system being controlled, the optimal value of this feedback gain differs depending on the conditions of use. In consequence, if the feedback gain K is a constant, then the feedback gain K may be optimal for some uses, but not for other uses. Thus, the feedback gain K may be excessively large and thereby cause hunting, etc., or may be excessively small and thereby deteriorate the response or controlling accuracy.
To obviate the above-described disadvantages, in Japanese Patent Laid-Open (Kokai) No. 191358/1983 for example, an equation (Vin=K1.times.PL'.times.D) is used to correct a line pressure PL. Specifically, this correction adjusts the control voltage Vin to the flow control valve in accordance with an output voltage Vout to a pressure control valve. The control voltage Vout to the pressure control valve is corrected in association with the control voltage Vin to the flow control valve, so that the flowrate Q, i.e., the speed ratio, can be suitably controlled, irrespective of the line pressure PL.
However, this system is not satisfactory because the effect is not precise.